The number 6 has a special significance. It represents the Law and Order and also associated with a Karmic Law of cause and effect. Number 6 is a perfect number, symmetrical in all parts, the number of the Soul.
Plums are ready they are not hard as a rock, soft just a little when pressed. I wouldn't use windfall, they are too ripe. If your making jam, it is best to use some underripe with the ripe ones. Last season I added small diced candied ginger to the plum jam. Really turned out nice.
Plum pudding can be made anywhere from a few weeks to a year in advance and allowed to ripen in a cool place. It must be steamed 2 hours before serving. Leave it in hot water until ready to serve with hard sauce. This recipe is from Entertaining, by Martha Stewart. Source: The Martha Stewart Show, December 2007